The WO 2009/112905 A2 discloses an optical biosensor in which an input light beam is totally internally reflected at a contact surface and the resulting output light beam is detected by a camera and evaluated with respect to the amount of target components at the contact surface. The target components comprise magnetic particles as labels, which allows to affect the processes in the sample by magnetic forces. In this and similar optical sensor devices, the imaged object plane is tilted with respect to the optical axis of the camera, which makes it difficult to evaluate its image.